1. Field of the Invention
The present invention relates to a belt type fixing device which is used to fix toner in an electrophotographic imaging apparatus such as a photocopier, printer, or facsimile.
2. Description of the Related Art
A conventional fixing device in an imaging apparatus typically used a pair of rollers to heat and press a recording sheet carrying toner. However, as color imaging apparatuses have spread, a belt type fixing device which heats the nip adequately and may be compact has been proposed.
In the belt type fixing device, usually a belt wound and put between two rollers is heated at a point away from the nip and the heat given to the belt is transferred to unfixed toner through circular movement of the belt. In this structure, a heat source need not be located at the nip, more specifically inside a fixing roller, as in the conventional fixing device, so a low-hardness elastic layer with a low thermal conductivity may be provided at the nip. The use of such a low-hardness elastic layer assures a wider nip.
One example of this type of belt type fixing device is disclosed in Japanese Patent Laid-open No. H08-137306. In this prior art, a fixing belt is put between two rotary rollers and an electromagnetic induction coil is located on the belt between the rollers to heat the belt directly. Since these rollers need circularly rotate the belt with a tensile force, they must have a relatively high strength and thus the rollers should have a relatively large thermal capacity. Hence, some of the heat of the belt spreads to the rollers. Since the rollers absorb some of the heat of the belt because of the large thermal capacity in this way, it is difficult to shorten the warming up time further even though the heating efficiency is improved by electromagnetic induction.
Another example is the method disclosed in Japanese Patent Laid-open No. H05-107961 whereby a heating body is in sliding contact with the outer surface of a heating roller in order to shorten the warming up time. Since the sliding contact surface is a fixing plane and vulnerable to scratching, there is the problem of low durability.
Also, there have been attempts to provide anon-rotary heater at the nip for direct heating. In this case, however, the heater must have both a heating function and a pressing function, so it is difficult to provide a wider nip and the heater holder and the heater must be manufactured with a high dimensional precision and accurately assembled, which increases the manufacturing cost.